The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, lowered costs etc. UMTS Terrestrial Radio Access Network (UTRAN) is the radio network of a UMTS system and evolved UTRAN (E-UTRAN) is the radio network of a LTE system. The radio base station in E-UTRAN is called evolved NodeB (eNB).
An E-UTRAN typically comprises user equipments (UE) 150 wirelessly connected to radio base stations 130a-c as illustrated in FIG. 1. The radio base stations 130a-c are directly connected to the core network (CN) 100. In addition, the radio base stations are also connected to each other. In contrast, in an UTRAN the radio base stations are connected to the CN via a Radio Network Controller (RNC), wherein each RNC controls the connected radio base stations.
Discontinuous reception (DRX) is a method used in wireless communication systems, e.g. LTE, to enable a reduced power consumption of the UE. In idle mode e.g., the UE and the network are coordinated in terms of having the same information about how and in what phases the data transfer happens. This means that during the other phases (when no data transfer happens) the UE receiver functionality can be switched off and can thus enter a low power state, hereinafter called sleep mode. FIG. 2 illustrates that the UE receiver only wakes up during short regularly occurring periods (once in each DRX cycle 202) known as paging occasions 201 to receive a sub frame. During these paging occasions the UE checks for paging messages in the sub frame.
FIG. 3 illustrates the basic downlink sub frame structure as defined in E-UTRAN. Each sub frame 300 of length 1 ms consists of two parts: A control part 301 during which Physical Downlink Control Channels (PDCCH) are transmitted, and a data part 302 during which Downlink Shared Channel (DL-SCH) are transmitted. Each PDCCH typically carries control information to a single UE when it is in active mode. This control information informs the UE how to receive and decode corresponding information on the Downlink Shared Channel (DL-SCH). What UE the PDCCH is directed to is indicated by a Radio Network Temporary Identifier (RNTI) included in the PDCCH.
The sub frame structure with its control and data fields is also assumed to be used for paging, but one difference is that a sub frame comprising paging messages can be directed to more than one UE. The time between paging occasions, i.e. the sleep-mode or DRX cycle, typically spans a large number of sub frames, e.g. 320 sub frames or a multiple of 320 sub frames. When a UE wakes up, it first receives and decodes the control part of the sub frame. If one of the PDCCHs in the control part includes an identifier indicating that the data part of the sub frame includes one or multiple paging messages, the UE proceeds to receive and decode the DL-SCH in the data part to find out if one of the paging messages was directed to this specific UE. The identifier in the control part can be described as a specific RNTI, which can be referred to as a Paging RNTI (P-RNTI). The P-RNTI is thus just an indication to all UEs addressed at that paging occasion that there are paging messages to read, but it does not say anything about to which UEs the paging messages are directed. If the UE determines that no paging message in the data part was directed to it, it can return to sleep mode again. If no P-RNTI is present in the control part of the sub frame, the UE can return to sleep mode immediately after decoding the control part, i.e. the UE does not need to read the data part of the sub frame.
Paging is capacity demanding for the system, as paging messages need to reach all UEs in a cell. The transmission of paging messages cannot be adapted for a specific UE as it is addressed to all UEs that listen for paging at the same paging occasion, and in the worst of cases UEs can be close to the cell border and thus far away from the radio base station (e.g. the eNB in E-UTRAN). To be sure to reach all UEs the transmit power needs to be set to a maximum (the level needed to reach UEs at the cell border), which means a low data rate and affects the number of paging messages that can be carried by one sub frame. The bigger the cell is, the more capacity demanding is the paging and the less amount of UEs can be paged in one sub frame.
If several UEs wake up from there sleep mode at the same time, i.e. if their paging occasions coincide, there might be a need to transmit multiple paging messages to the different UEs in the same sub frame. However, especially in case of narrowband transmissions with limited data rates on the Downlink Shared Channel (DL-SCH), it may not be possible to include all these paging messages within the data part of a single sub frame. This situation is hereafter referred to as overflow. An overflow situation is handled by waiting for the next paging occasion to transmit the rest of the paging messages. Overflow thus implies a delay in the paging.
To minimize the delay for paging, a network operator can adapt the DRX cycle. With a shorter DRX cycle the delay at overflow will be shorter, as the time to the next paging occasion is shorter. Another aspect is that a shorter DRX cycle will give more paging occasions per time unit which leads to fewer paging messages per paging occasion and thus a lower risk for overflow. The disadvantage of shorter DRX cycles is an increased UE power consumption as the UE has to wake up for paging more often.